Part of checking in is dropping off luggage. While you can get your BP up to 24hrs early online, this is only one portion of checking in. Until a transporter is invented you'll have to complete checking in with bags at the airport.

What if they had been able to leave ontime? This is why you have to be fully checked in and ready to board at scheduled departure. Also outside the US most airport checkin is contract. They close at end of scheduled checkin and they may not even be there because they have to start on the next airlines flight handling.